The present invention relates to a method of storing binary optical information and, more particularly, to the use of photoactivated shifts in the phase transition between the ferroelectric (FE) and the antiferroelectric (AFE) phases within a polycrystalline PLZT (lead lanthanum zirconate titanate) ceramic or film containing device to store and erase binary optical information.
A photoferroelectric optical information storage device which uses the field-induced AFE-to-FE phase transistor in AFE-phase PLZT compositions to store high-resolution, high-contrast optical information was described in "Photoferroelectric Image Storage in Antiferroelectric-Phase PLZT Ceramics," by Cecil E. Land, IEEE Trans. on Electron Device, Vol. ED-26, No. 8, 1143-1147 (1979). In this device, the electric-field-induced AFE-to-FE phase transition is photoinhibited by exposure to near-UV light with photon energies equal to or greater than the band gap (about 3.4 eV). This storage device employs an intrinsic photoferroelectric effect similar to that occuring in FE-phase compositions. This photoferroelectric effect in the AFE-phase material provides a basis for erasable optical information storage such as photographic images with high resolution and contrast or holograms with large diffraction efficiencies. In unimplanted AFE-phase PLZT, the intrinsic photoferroelectric effect relies on photon absorption to photoexcite carriers into the conduction band and thereby to increase the threshold voltage for field-inducing the AFE-to-FE phase transition.
Both resolution and contrast of stored photographic images and the diffraction efficiency of stored holograms are substantially higher for storage in AFE-phase material than for comparable photoferroelectric storage in FE-phase compositions. The absence of domain structure in the AFE-phase removes a major source of light scattering and thereby increases contrast and/or diffraction efficiency of stored information.